1. Field of the Invention
The present invention relates to a combustion control apparatus which controls the combustion in a gas burner which is contained inside a gas appliance such as a gas-operated water heater or the like.
2. Description of Related Art
As this kind of combustion control apparatus, there has hitherto been known the following. Namely, a solenoid-operated gate valve is interposed as a safety valve in a gas supply passage which supplies a gas burner with gas. When an operation for igniting the gas is performed, a microcomputer recognizes the ignition operation. A solenoid of the gate valve is then supplied with electric current, thereby opening the gate valve. In this kind of combustion control apparatus in which the combustion is controlled with the microcomputer, the following is necessary. Namely, if the ignition is not confirmed by a flame rod which is provided in the neighborhood of a flame peep hole in the gas burner, even after the lapse of a predetermined time from the start of ignition, the gate valve must be closed. In addition, also in case the flame of the gas burner has failed for some reasons after the gas was once ignited, the gate valve must similarly be closed. An output signal of the flame rod is constantly monitored by the microcomputer. In case the microcomputer judges that the ignition has not taken place after the lapse of a predetermined time from the start of ignition, or in case the microcomputer judges that the flame of the gas burner has failed in the course of combustion, the microcomputer shuts off the electric current which is being supplied to the solenoid of the gate valve, thereby closing the gate valve.
In this manner, since the microcomputer operates especially to close the gate valve, there is the following disadvantage. Namely, even in case the gate valve must be closed because the signal from the flame rod does not attain the ignited state, there will arise a state in which the gate valve is not closed if the microcomputer runs away out of control. As a solution, there is provided a watchdog timer circuit (WDT circuit) so that, when an abnormal condition such as the microcomputer""s runaway or the like occurs, the gate valve is forcibly closed by the watchdog timer circuit.
The conventional watchdog timer circuit functions to judge the presence or absence of a signal. In other words, a signal is outputted to the watchdog timer circuit at a predetermined cycle. As long as the signal is being outputted from the watchdog timer circuit, the watchdog timer does not output a reset signal on the assumption that the microcomputer is operating normally. Instead, when the signal from the microcomputer is stopped due to the microcomputer""s runaway or the like, the watchdog timer circuit outputs the reset signal to the microcomputer, thereby resetting the microcomputer. Once the microcomputer has been reset, the gate valve is closed because the electric current is forcibly stopped even if it is being supplied to the solenoid of the gate valve. The supply of the gas to the gas burner is thus stopped.
The microcomputer is provided with an oscillator section such as a crystal oscillator or the like. The operation speed of the microcomputer is dependent on the oscillation frequency of the oscillator section. Should the oscillation frequency vary for some reasons, the operation speed of the microcomputer also varies in a manner interlocked with the change in the oscillation frequency. For example, if the oscillation frequency becomes lower by 40%, the time for the timer inside the microcomputer to count becomes longer by 40%. As an example, in case the gate valve is so programmed as to be closed if ignition does not take place during a period of time of 10 seconds from the start of ignition, there is a case in which 14 seconds has actually passed from the time of ignition to the time of closing the gate valve, even if the gate valve is closed after a lapse of 10 seconds as counted by a clock inside the microcomputer.
However, the watchdog timer circuit used in the above-described conventional combustion control apparatus judges only as to whether a signal from the microcomputer is being outputted or not. Therefore, in a state in which the signal is being outputted to the watchdog timer circuit even though the oscillating frequency has varied, the watchdog timer circuit cannot detect the abnormal condition. As a result, the microcomputer will not be reset.
In view of the above-described disadvantages, the present invention has an object of providing a combustion control apparatus in which the gas supply to the gas burner is stopped also in case the oscillating frequency deviates, aside from the case of the microcomputer""s runaway, or the like.
In order to attain the above and other objects, the present invention is a combustion control apparatus comprising: a first closing means for closing a gas supply passage which supplies a gas burner with gas, thereby stopping gas supply to the gas burner; a microcomputer for operating the first closing means on a predetermined occasion; and a watchdog timer circuit for monitoring an operating state of the microcomputer based on a pulse signal which is outputted from the microcomputer, wherein, when a frequency of the pulse signal which is outputted from the microcomputer to the watchdog timer circuit deviates from a predetermined frequency range which is based on a reference frequency, the first closing means is operated as an abnormal condition of the microcomputer, thereby stopping the gas supply to the gas burner, even in a state in which the pulse signal is being outputted.
In the conventional watchdog timer circuit, as long as a signal is being outputted from the microcomputer, the microcomputer is judged to be normal. Therefore, when the operation speed of the microcomputer has lowered, it is impossible to take any correcting measures. In the above-described arrangement according to the present invention, on the other hand, an arrangement has been made that the gas supply to the gas burner is stopped when the frequency of the pulse signal deviates from the above-described predetermined frequency range. Therefore, the unburned gas can be prevented from being discharged out of the gas burner for more than an originally scheduled period of time.
It is also possible that the watchdog timer itself gets out of order. In such a case, it is preferable to make an arrangement that the microcomputer outputs into the watchdog timer circuit a pulse signal which is away from the predetermined frequency range when a combustion control is started, thereby checking the watchdog timer circuit.
In case the pulse signal to be outputted from the microcomputer deviates from the predetermined frequency range, the gate valve is closed by resetting the microcomputer, or the like operation. Preferably, the combustion control apparatus further comprises a second closing means for directly stopping the gas supply by the watchdog timer circuit without an operation of the microcomputer. The second closing means is then operated when the pulse signal deviates from the predetermined frequency range. In this arrangement, the gas supply to the gas burner can be stopped in a surer way.